1. Field of the Invention
The present invention generally relates to an apparatus and method for feeding back channel quality information and performing scheduling using the fed-back channel quality information in a wireless communication system. More particularly, the present invention relates to an apparatus and method for feeding back channel quality information and performing scheduling using the fed-back channel quality information in a wireless communication system based on Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
Conventionally, a wireless communication system performs communication through a radio channel between mobile stations (MSs) or between an MS and a base station (BS) of a predetermined network. The wireless communication system was initially developed to provide a voice service, but has been advanced to provide a data service in response to user requests. Technologies are needed which can efficiently transmit data due to an increase in an amount of data to be transmitted and an increase in the number of users. According to this need, wireless communication systems transmit user-by-user data by correctly detecting channel situations between the BS and the MSs.
In a method for detecting the channel situations between the BS and the MSs, each MS measures channel quality of a signal received from the BS and feeds back channel quality information or a channel quality indicator (CQI). For example, in a typical mobile communication system using Orthogonal Frequency Division Multiple Access (OFDMA), each MS measures the signal strength of a pilot channel received from the BS and transmits information about the measured strength to the BS. Then, the BS can detect a channel situation between the BS and an associate MS from the reception strength of the pilot channel. Thus, the BS efficiently transmits data by employing the detected channel situation for scheduling and power control of a forward transmission.
As described above, wireless communication systems are being developed into systems capable of accommodating an increased number of users and transmitting a large amount of data. However, the OFDMA mobile communication system based on the current voice service has a limitation in transmitting a large amount of data at a high rate. Thus, research is being actively conducted on other types of systems rather than the OFDMA system.
One system for transmitting a large amount of data at a high rate is a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). The OFDM is a type of Multi-Carrier Modulation (MCM) scheme for converting a serially input symbol stream, to be transmitted to a user, to parallel form, modulating parallel data in a plurality of orthogonal subcarriers, in other words, a plurality of subcarrier channels, and transmitting the subcarrier channels. A scheme for identifying multiple users through the OFDM is OFDMA. A method for configuring a channel to transmit one data packet in the OFDMA system is divided into an Adaptive Modulation & Coding (AMC) transmission scheme and a diversity transmission scheme. The AMC transmission scheme configures one physical channel by combining adjacent subcarriers and adjacent symbols, and is referred to as a localized transmission scheme. On the other hand, the diversity transmission scheme configures one physical channel by combining scattered subcarriers and is referred to as a distributed transmission scheme.
First, a method for allocating orthogonal frequencies to users and a transmission method in the OFDMA mobile communication system will be described with reference to the accompanying drawings.
FIG. 1A illustrates an example of allocating orthogonal frequency resources to users in the OFDMA mobile communication system, and FIG. 1B illustrates another example of allocating orthogonal frequency resources to users in the OFDMA mobile communication system.
In FIGS. 1A and 1B, the horizontal axis represents time and the vertical axis represents orthogonal frequencies. As illustrated in FIG. 1A, multiple orthogonal frequency resources can form one subcarrier group and subcarrier groups are allocated to one communication MS. Further, the subcarrier groups are transmitted during at least one OFDM symbol time. In FIGS. 1A and 1B, reference numeral 101 denotes one subcarrier and reference numeral 102 denotes one OFDM symbol. As illustrated in FIG. 1A, subcarrier groups 103, . . . , 104 are included in a reallocation period 105 for reallocating frequency resources.
An example of allocating frequency resources will be described with reference to FIGS. 1A and 1B.
FIG. 1A illustrates an example of transmitting data using AMC technology in the OFDMA system. As illustrated in FIG. 1A, a total frequency band is conventionally divided into N subcarrier groups or sub-bands in the OFDM system using AMC and performs AMC operations on a subcarrier group-by-subcarrier group basis. Hereinafter, one subcarrier group is referred to as one AMC sub-band. That is, Subcarrier Group 1 denoted by reference numeral 103 is referred to as “AMC Sub-Band 1”, and Subcarrier Group N denoted by reference numeral 104 is referred to as “AMC Sub-Band N”. In the conventional system, scheduling is performed in a unit of multiple OFDM symbols as indicated by reference numeral 105. As described above, the conventional OFDM system independently performs AMC operations on multiple AMC sub-bands. Thus, each MS feeds back CQI information on a sub-band-by-sub-band basis. The BS receives channel quality information of sub-bands to schedule the sub-bands and transmits user data on the sub-band-by-sub-band basis. In an example of the scheduling process, the BS selects MSs of the best channel qualities on the sub-band-by-sub-band basis and transmits data to the selected MSs, such that system capacity can be maximized.
According to characteristics of the above-described AMC operation, it can be seen that a good situation is the case where multiple subcarriers for transmitting data to one MS are adjacent to each other. This is because channel response strengths relating to adjacent subcarriers may be similar to each other but channel response strengths relating to far away subcarriers may be significantly different from each other when frequency selectivity occurs in a frequency domain due to a multipath radio channel. The above-described AMC operation maximizes system capacity by selecting subcarriers relating to good channel responses and transmitting data through the selected subcarriers. Therefore, it is preferred that a structure can select multiple adjacent subcarriers relating to good channel responses to transmit data through the selected adjacent subcarriers. The above-described AMC technology is suitable for a data transmission to a particular user. It is not preferred that channels to be transmitted to multiple users, for example, broadcast or common control information channels, are adapted to a channel state of one user.
FIG. 1B illustrates an example of transmitting user data using diversity technology in the OFDMA system. As illustrated in FIG. 1B, it can be seen that subcarriers carrying data to be transmitted to one MS are scattered, which is different from the AMC mode of FIG. 1A. The diversity transmission is suitable for the case where a transmission of a combination of data of one user in a particular sub-band is not easy because a data transmitter cannot know a channel state. The diversity transmission is also suitable for a channel to be transmitted to unspecified users as in broadcasting.
The above-described OFDMA wireless communication system conventionally transmits packet data. The system for transmitting the packet data has the structure of FIG. 2. FIG. 2 is a conceptual diagram illustrating a relation between a BS or Access Point (AP) and MSs or Access Terminals (ATs) in a wireless communication system for performing packet data communication.
Referring to FIG. 2, MSs or ATs 211, 212, 213, 214, and 215 communicate with the BS or AP 200 through a predetermined channel. The BS or AP 200 transmits a predetermined reference signal, for example, a pilot signal. The MSs or ATs 211˜215 measure the strength of a signal received from the BS 200 and feed back information about the measured strength to the BS or AP 200, respectively. Thus, the BS or AP 200 performs scheduling using information about strengths of signals received from the MSs or ATs and transmits data to the MSs or ATs. In FIG. 2, the arrows from the BS or AP 200 to the MSs or ATs 211˜215 are signals transmitted on forward channels and the arrows from the MSs or ATs 211˜215 to the BS or AP 200 are signals transmitted on reverse channels.
As described with reference to FIG. 2, a mobile communication system for performing packet data communication widely employs a scheme in which an MS measures the quality of a forward channel and feeds back channel quality information to the BS, because a transmitter of the BS can easily select a suitable data transmission rate according to a channel state when knowing a forward channel state.
A scheme for feeding back forward channel quality information from an MS in the OFDMA mobile communication system will be described.
FIG. 3 is a timing diagram illustrating an operation for feeding back forward channel quality information from an MS in the OFDMA mobile communication system for performing packet data communication.
Referring to FIG. 3, blocks 301, 302, 303, and 304 indicate that the MS feeds back forward channel quality information in a block unit. The channel quality information is fed back during one feedback information transmission. In the OFDMA system, each MS conventionally feeds back a pair of a sub-band index and channel quality information. That is, a sub-band index and its mapped channel quality information are fed back such that sub-band-by-sub-band channel quality information is fed back. Because a large number of sub-bands are conventionally present in the OFDMA wireless communication system, severe reverse load occurs when channel quality information of all sub-bands is fed back. Thus, the MS conventionally selects several best sub-bands and feeds back sub-band indices and their channel quality information.
Because the number of sub-bands capable of being allocated from the BS to the MS is reduced when the number of feedback sub-bands is reduced, the forward performance is degraded. For example, when the MS selects only one sub-band and feeds back a sub-band index and its channel quality information to the BS, the BS can allocate only the sub-band selected by the MS. If the sub-band cannot be allocated to the MS, the forward performance for the MS is degraded.
When the number of feedback sub-bands increases, an increase in the reverse load is caused and therefore the reverse throughput is degraded. In contrast, when the number of feedback sub-bands decreases, the forward channel selectivity is reduced and therefore the forward performance is degraded. Thus, a need exists for an improved method capable of performing processing such that a relation between the reverse load and the forward channel selectivity is appropriately established.